1. Field of the Invention
The present invention relates generally to a technique to improve emitter tip quality on large area passive matrix cold cathode field emission displays and, in particular, to enhance electron emission from the emitter tips.
2. Description of Related Art
Cathode ray tube (CRT) displays are commonly used in display devices such as televisions and desk-top computer screens. CRT displays operate as a result of a scanning electron beam from an electron gun striking phosphors resident on a distant screen. The electrons increase the energy level of the phosphors. When the phosphors return to their original energy level, they release photons which are transmitted through the display screen (normally glass) forming a visual image to a person looking at the screen. A colored CRT display utilizes an array of display pixels wherein each individual display pixel is comprised of a trio of color generating phosphors (that is, each pixel is split into three colored parts, which alone or in combination create colors when activated). Color images are created by exciting the appropriate colored phosphors.
Flat panel displays are becoming increasingly popular to display the information of computer systems and other devices. Typically, flat panel displays are lighter and utilize less power than conventional CRT display devices.
One type of flat panel display is known as a cold cathode field emission display (FED). Cold cathode FED's are similar to CRT displays in that they use electrons to illuminate a cathodoluminescent screen. The electron gun is replaced with numerous (at least one per display pixel) emitter sites. When activated by a high voltage, the emitter sites release electrons which strike the display screen's phosphor coating.
FED technology utilizes a matrix addressable array of pointed, thin film, cold field emission cathodes in combination with a phosphor luminescent screen. U.S. Pat. No. 4,940,916, which is hereby incorporated by reference in its entirety, discloses an electron source, with micropoint emissive cathodes, and a display by use of cathodoluminescent excited by field emission from the electron source. Each cathode has an electrically conductive layer, a continuous resistive layer on the conductive layer and a patterned array of a plurality of micropoints. The display includes a cathodoluminescent anode facing the source.
A further example of FED technology can be found in U.S. Pat. No. 5,210,472, the disclosure of which is incorporated herein by reference. An emissive flat panel display operates on the principles of cathodoluminescent phosphors excited by cold cathode field emission electrons. A face plate having a cathodoluminescent phosphor coating receives patterned electron bombardment from an opposing base plate thereby providing a light image which can be seen by a viewer. The face plate is separated from the base plate by a vacuum gap and, in some embodiments, the two plates are prevented from collapsing together by physical standoffs or spacers fixed between them.
The base plate of a field emission display is comprised of arrays of emission sites (emitters) which are typically sharp-tipped pyramids that produce electron emission in the presence of an intense electric field. An extraction grid within a face plate of the field emission display is disposed above the sharp emitters and provides the intense positive voltage for the electric field and a mechanism for addressing and activating the generation of electron beams from those sites. Varying the charge which is delivered to the phosphor in a given pixel from an emission array will vary the light output (brightness) of the pixel associated with it. Two techniques for varying the charge delivered by an emission array are to either vary the time period of activation (duty cycle) or to vary the emission current.
The sharp pyramids that make up the arrays of emission sites are typically formed of silicon (Si) and are covered with a metallic film. The emission sites need to maintain a sharp profile to emit electrons in a reliable and controlled manner. Accordingly, there is a desire and need for an emission site and a method of forming an emission site having a tip which is able to maintain a sharp profile.
Producing an emission site having a sharp profile is difficult due to the nature of the silicon-to-metal interface and the grain size of the metal used to coat the pyramids of silicon. Accordingly, there is a desire and need to produce emission sites having a tip capable of maintaining a sharp profile in an easy manner.